In an automotive internal combustion engine, the friction between a cam lobe and its opposing member, i.e. a valve lifter or a shim attached to an end of the valve lifter, accounts for 20% of the total engine mechanical loss especially in a low engine rotation speed range including idling speed. The technique of reducing the friction between the cam lobe and the lifter/shim is important to provide a direct improvement of vehicle fuel efficiency.
The cam lobe and the lifter slide against each other at the highest degree of surface pressure in the internal combustion engine. Mechanically, there is a moment at which a lubricant film does not exist between the cam lobe and the lifter. It can be thus said that the state of lubrication is extremely severe. In order to reduce the friction between the cam lobe and the lifter, it is effective to reduce direct contact (metal contact) between the cam lobe and the lifter and improve the state of lubrication by smoothing the surface roughness of both of the cam lobe and the lifter as well as to reduce the friction on metal contact by surface treating with e.g. a solid lubricant or by using an additive.
Based on the above facts, it has recently become common to smooth the surface roughness of a sliding portion of the lifter and then apply a hard thin film of e.g. titanium nitride (TiN) or chromium nitride (Cr2N) to the sliding portion, or coat the sliding portion with a resinous material in which a solid lubricant of molybdenum disulfide (MOS2) is dispersed.
By the way, the greatest merit of hard thin films formed by a PVD (Physical vapor deposition) method and a CVD (chemical vapor deposition) method is to be able to attain considerably high surface hardness as compared with that attained by another surface treating e.g. plating or by surface hardening e.g. heat treatment. It is expected that the application of such a hard thin film to the sliding member will provide more significant improvement in the wear resistance of the sliding member.
Under lubrication, the hard thin film can prevent the sliding member from wearing to deteriorate its surface roughness. This makes it possible to prevent the sliding member from wearing away its opposing member due to such a deterioration of surface roughness, to prevent the friction between the sliding member and the opposing member from becoming increased by their increased direct contact (metal contact), and to maintain the lubrication at the initial state over a long time period. Also, the hard thin film can be expected to provide the effect of achieving smoothed surface roughness due to its high hardness upon contact with the opposing member and to thereby improve the state of lubrication by the smoothed surface roughness of both of these members.
Further, it is known that amorphous carbon films such as a diamond-like carbon (DLC) film, which are one kind of hard thin film, have the property of a solid lubricant in addition to high hardness and show low friction coefficients under no lubrication.
In microscopic analysis, the contact of the sliding member with the opposing member under lubrication includes a region in which the sliding member slides against the opposing member via a lubricant film and a region in which the sliding member and the opposing member make direct contact (metal contact) between peaks of their surface roughness. It is expected that the application of the DLC film to the above-mentioned direct contact region will allow reduction in the friction without lubrication. In recent years, the application of the DLC film to the sliding member has been thus examined as a low-friction technique for the internal combustion engine.
However, the hard thin film formed by the PVD or CVD method has higher internal stress and much higher hardness than that formed by another surface treating e.g. plating. When being applied to the sliding member of a mechanical part, the hard thin film often presents such problems that the film becomes separated from its base and becomes cracked. In order to prevent the separation of the film, it has been proposed that an intermediate layer is provided to improve the adhesion between the film and the base and that the film is formed in a multilayer structure to lessen the internal stress of the film.
There are few proposals for preventing the cracking of the film and the separation of the film resulting from the film cracking by controlling the surface roughness and profile of the hard thin film, notably hard carbon film, as well as the surface roughness and profile of the opposing member.
One of the proposals is to control the roughness of the opposing member i.e. the cam (Japanese Laid-Open Patent Publication No. JP11-294118A). This proposal is based on the principle that the input of load to the film can be controlled to prevent the separation of the film by limiting the roughness of the cam to a given value or smaller.
Another proposal is to control the surface profile of the film, more specifically to control the height and amount of macro particles (droplets) remaining on the surface of the film formed by arc ion plating (Japanese Laid-Open Patent Publication No. JP7-118832A).
As a result of extensive researches, the inventors have found that, when the hard carbon film is formed by PVD arc ion plating on a lifter part, the film cannot always obtain sufficient durability in the above proposed methods.
In the proposal of controlling the roughness of the opposing member i.e. the cam (JP11-294118A), if the surface roughness of the hard carbon film formed by arc ion plating exceeds a given value, the film can be damaged from sliding and be separated due to the film damage without regard to the roughness of the cam.
In the proposal of controlling the surface profile of the film (JP7-118832A), the height and amount of macro particles (droplets) remaining on the film formed by arc ion plating are controlled to within certain limits so as to smooth the surface roughness of the opposing member in a short time and thereby prevent the opposing member from being ground too much. The above proposal is not designed through consideration of a detailed mechanism of film separation. The separation of the film can thus occur depending on the surface roughness and profiles of the film and of the opposing member, even when the surface profile of the film is within the certain limits. It has also become apparent from the researches made by the inventors that the hard carbon film needs to be controlled closely according to its film quality, because the hard carbon film is more likely to be brittle than the previously known films such as a TiN film and a CrN film.
For the reasons above, it is evident that the expected effects of the hard carbon film cannot be obtained sufficiently in the conventional proposals.